1. Field of the Invention
This invention relates to the production of sodium nitrite and more particularly to the recovery of sodium nitrite from aqueous sodium nitrite solutions containing sodium chloride impurities.
2. Description of the Prior Art
It is well known that if nitrous gases, i.e. gases containing nitric oxide and nitrogen dioxide, are brought into contact with alkaline aqueous solutions, the nitric oxide and nitrogen dioxide will be absorbed with the formation of nitrites and nitrates. See U.S. Pat. Nos. 1,903,815 and 1,978,431. Such gases are typically obtained from a wide variety of sources, as for example, by the arc process of nitrogen fixation, by the air-oxidation of ammonia or as waste gases from an absorption process of which nitric acid is the primary product. By controlling the ratio of nitric oxide and nitrogen dioxide so that more nitric oxide than nitrogen dioxide is present in the gas feed, the amount of nitrates that is formed can be minimized, thereby allowing maximum conversion of the nitrogen oxide to the nitrites.
In such processes nitrous gases are contacted for absorption in one or more stages with a liquid medium maintained alkaline by the presence of suitable alkaline reacting agents, e.g. sodium hydroxide, sodium carbonate, sodium bicarbonate and mixtures thereof. The liquor, herein termed the "final absorption liquor", which is separated from the last absorption stage contains dissolved sodium nitrite together with sodium nitrate and minor amounts of unreacted alkaline reacting agent. Further, use of a nitrous gas feed containing carbon dioxide will also cause the finished absorption liquor to contain sodium carbonate/bicarbonate values due to the absorption of CO.sub.2 in the aqueous medium.
The final absorption liquor is passed on to an evaporator crystallizer for evaporation of at least a portion of the water content of the final absorption liquor, thereby producing a liquor, herein termed the "draw slurry", containing sodium nitrite solids. The draw slurry is then passed to a solid separator to remove sodium nitrite crystals substantially free of other components of the liquor as product. The mother liquor (herein termed the "draw slurry mother liquor") which is obtained following the separation of the product sodium nitrite crystals from the draw slurry, still contains substantial amounts of dissolved sodium nitrite. Typically, a portion of this liquor is recycled to the absorption stage and/or evaporator for recovery of additional sodium values and the balance is discarded to purge impurities from the system and thereby prevent their build-up in the evaporator and their consequent co-precipitation as impurity with the sodium nitrite crystals.
While such prior art processes enable substantially pure sodium nitrite crystals to be obtained, cheaper sources of alkaline reacting material, such as 50 percent sodium hydroxide solutions derived from electrolytic cell production of chlorine, require the purging of large amounts of draw slurry mother liquor to avoid an undesirable build-up of chloride ions in the evaporator and the consequent chloride contamination of the sodium nitrite crystals therein produced. For example, it has been found that if the draw slurry mother liquor contains greater than 5 weight percent sodium chloride, sodium chloride will coprecipitate as impurity in the sodium nitrite solids in the evaporator. Thus, the portion of the draw slurry mother liquor recycled to the absorption stage or to the evaporator should contain less than about 5 weight percent, preferably less than about 4.5 weight percent, sodium chloride so as to substantially minimize sodium chloride precipitation which greatly complicates recovery of substantially pure sodium nitrite crystals. While purging draw slurry mother liquor would be helpful in lowering the amount of sodium chloride values recycled to the absorber or evaporator, discarding substantial volumes of mother liquor is undesirable since it results in the loss of significant amounts of sodium nitrite which is dissolved in the separated draw slurry mother liquor.
While the prior art is aware of theoretical solubilities in various aqueous systems containing such salts as sodium nitrite, sodium chloride, sodium nitrate or sodium carbonate, no prior art process has been developed for removal of sodium chloride and other impurities from a 5-component aqueous system containing the sodium salts, e.g. NaNO.sub.2, NaNO.sub.3, NaCl and Na.sub.2 CO.sub.3. See, e.g., the following references: J. Bureau, Anales de Chemise, 78, 51-55, 103-105 and 122-139 (July and August 1937); I. N. Kuzminykh and E. L. Yakhontova, "The Solubility of Sodium Nitrite and Nitrate in Water when Soda is Present", J. Applied Chemistry U.S.S.R., 24(1), 199-202 (1951); E. Erdo's and H. Simkova "Solubility of Electrolytes. The Ternary System Sodium Nitrite - Sodium Chlorine - Water ", Collection of Czechoslav. Chem. Commun., 24, 503-507 (1959); E. Erdo's and H. Simkova, "The solubility of Electrolytes. The Quaternary System Sodium Nitrate - Sodium Nitrite - Sodium Chloride - Water", Collection of Czechoslav. Chem. Commun., 24, 694-699 (1959); and Mellor' s Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VIII, Supplement II, N (Part II), pp 370-375 (John Wiley & Sons, Inc. 1967).